simplify: Minor refactoring of performEdgeCollapses

To avoid mistakes, extract kind into a variable (we're only concerned
with source vertex kind for classification since that's what determines
the wedges we need to remap), and also add a few comments.

Notably, we probably don't need to worry about attribute quadrics for
complex vertices long term; the intention for complex classification has
been to treat vertices with attributes that are similar enough as one,
so this probably can function similarly to ranking where we're okay with
ignoring the topology somewhat.

Also, while we're here, adjust complex collapses to collapse to i1, not
r1. This code is not used right now but collapsing to r1 feels slightly
wrong since in some cases we'd be collapsing to a vertex that was never
connected to any wedge in the complex.

Author: zeux

src/simplifier.cpp

@@ -1128,6 +1128,8 @@ static size_t performEdgeCollapses(unsigned int* collapse_remap, unsigned char*
 		unsigned int r0 = remap[i0];
 		unsigned int r1 = remap[i1];
 
+		unsigned char kind = vertex_kind[i0];
+
 		// we don't collapse vertices that had source or target vertex involved in a collapse
 		// it's important to not move the vertices twice since it complicates the tracking/remapping logic
 		// it's important to not move other vertices towards a moved vertex to preserve error since we don't re-rank collapses mid-pass
@@ -1160,26 +1162,29 @@ static size_t performEdgeCollapses(unsigned int* collapse_remap, unsigned char*
 			quadricAdd(attribute_quadrics[i1], attribute_quadrics[i0]);
 			quadricAdd(&attribute_gradients[i1 * attribute_count], &attribute_gradients[i0 * attribute_count], attribute_count);
 
-			if (vertex_kind[i0] == Kind_Seam)
+			// note: this is intentionally missing handling for Kind_Complex; we assume that complex vertices have similar attribute values so just using the primary vertex is fine
+			if (kind == Kind_Seam)
 			{
+				// seam collapses involve two edges so we need to update attribute quadrics for both target vertices; position quadrics are shared
 				unsigned int s0 = wedge[i0], s1 = wedge[i1];
 
 				quadricAdd(attribute_quadrics[s1], attribute_quadrics[s0]);
 				quadricAdd(&attribute_gradients[s1 * attribute_count], &attribute_gradients[s0 * attribute_count], attribute_count);
 			}
 		}
 
-		if (vertex_kind[i0] == Kind_Complex)
+		if (kind == Kind_Complex)
 		{
+			// remap all vertices in the complex to the target vertex
 			unsigned int v = i0;
 
 			do
 			{
-				collapse_remap[v] = r1;
+				collapse_remap[v] = i1;
 				v = wedge[v];
 			} while (v != i0);
 		}
-		else if (vertex_kind[i0] == Kind_Seam)
+		else if (kind == Kind_Seam)
 		{
 			// remap v0 to v1 and seam pair of v0 to seam pair of v1
 			unsigned int s0 = wedge[i0], s1 = wedge[i1];
@@ -1200,7 +1205,7 @@ static size_t performEdgeCollapses(unsigned int* collapse_remap, unsigned char*
 		collapse_locked[r1] = 1;
 
 		// border edges collapse 1 triangle, other edges collapse 2 or more
-		triangle_collapses += (vertex_kind[i0] == Kind_Border) ? 1 : 2;
+		triangle_collapses += (kind == Kind_Border) ? 1 : 2;
 		edge_collapses++;
 
 		result_error = result_error < c.error ? c.error : result_error;